This is a revised R01 proposal to address the biochemistry and function of two novel, leukocyte restricted signaling molecules. Fgd2 and Fgd3 are guanine nucleotide exchange factors (GEF) for the Ras homology (Rho) GTPases. They have significant sequence identity with known Cdc42 GEFs and are members of the Dbl homology family of GEFs. The Fgd subfamily includes 6 members and is distinguished by inclusion of FYVE domains that in other systems promote phospholipid binding and vesicular targeting. RhoGEFs play a number of crucial roles in receptor signaling, vesicle trafficking, cytoskeletal regulation, migration and the regulation of cell growth. Prior to our work, Fgd2 had not been characterized as a protein. Fgd2 is expressed at high levels in B cells and in macrophages. In B cells, Fgd2 expression is regulated by antigen receptor signaling and is expressed in memory B cells. In some contexts Fgd2 appears to be regulated reciprocally with a distinct family member, Fgd3. We have recently generated a conditional knockout of Fgd2. In this proposal the roles of Fgd2 and Fgd3 are addressed through the following three Specific Aims. 1) To characterize the biological functions of Fgd2 in the contexts of genetic knockouts and overexpression. 2) To characterize biochemical properties and specificities of Fgd2 and the related molecule Fgd3. A structure/function analysis will be carried out to determine the roles of individual motifs, to generate mutant forms, and to assess Fgd2 posttranslational modifications and interacting proteins. 3) To generate and analyze Fgd3 deficient mice and to address the issue of possible compensation of Fgd2 function by Fgd3. The long term goal of these studies is to understand how B cell development and antibody responses are regulated at the molecular level.